Two vehicles surround a trailer (7)
I believe the answer is:
caravan
'trailer' is the definition.
(caravan is a kind of trailer)
'two vehicles surround a' is the wordplay.
'two vehicles' means two substitutions for 'vehicle'.
'vehicle' becomes 'car' (car is a kind of vehicle).
'vehicle' becomes 'van' (type of vehicle).
'surround' is an insertion indicator.
'car'+'van'='carvan'
'carvan' going around 'a' is 'CARAVAN'.
(Other definitions for caravan that I've seen before include "Trailer with living quarters" , "Group travelling together across a desert in single file" , "Towed vehicle" , "Towable home" , "Holiday vehicle" .)